Mix of beef bones - saved from previous batch of bone broth
Enough water to cover bones
2 tbsp of apple cider vinegar
Instructions
Place bones in slow cooker
Add enough water to just cover the bones
Add cider vinegar - or sub another acid - I've used lemon juice in a pinch
Turn the slow cooker on low and set for the longest time option - mine goes only to 10 hours, I simply reset it a couple of times over about 30 hours. It's perfectly safe to let it go up to 48 hours.
Optional: if you use veggies to flavor the broth, put them in when there are about 6 total hours left. If you use herbs wait until the last hour to half hour of cook time, otherwise the broth will end up tasting bitter.
Notes
I've heard that if you let the bones sit in water for an hour with your acid of choice that it helps to draw out more minerals from the bones. I have not tasted a difference or seen data supporting this which is why I don't recommend it. If you have data to share supporting this please do!
Recipe by Nosh Nerd at https://www.noshnerd.com/recipes/gaps/beef-stock-reuse-those-bones/